A mayenite type compound has a typical composition of 12CaO.7Al2O3(hereinafter referred to also as “C12A7”) and a characteristic crystal structure having three-dimensionally linked voids (cages) with a diameter of about 0.4 nm. The framework forming the cages has a positive charge, and 12 cages are formed in a unit cell. And, one sixth of the cages are occupied by oxygen ions so that an electrically neutral condition of the crystal is satisfied, but these oxygen ions have chemically different properties from those of the other oxygen ions constituting the framework and are thus particularly called “free oxygen ions”. For this reason, the C12A7 crystal is denoted as [Ca24Al28O64]4+.2O2− (Non-patent Document 1).
A powder of C12A7 crystal or its sintered body may be subjected to heat treatment in a reducing atmosphere to let electrons be included in the cages, thereby to impart permanent conductivity at room temperature (Patent Document 1). These included electrons are loosely bound to the cages and can freely move in the crystal, whereby conductivity is imparted to the mayenite type compound (Patent Document 2).
Further, a powder of C12A7 crystal or its sintered body may be subjected to heat treatment in an atmosphere of an inert gas containing hydrogen to let H− be included in the cages. While H− is included in the cages, it is irradiated with an ultraviolet ray, electron beam or a plasma, whereby electrons are included in the cages by a reaction of H−→H0+e−. These included electrons are loosely bound to the cages and can freely move in the crystal, whereby permanent conductivity can be imparted to the mayenite type compound at room temperature (Patent Document 3). The sintered body may be properly masked before the irradiation with an ultraviolet ray etc. in order to obtain a fine wiring pattern easily.
Since such a mayenite type compound has a high reducing property, electrons are likely to be discharged from the cages at high temperature as compared with free oxygen ions. For this reason, when a mayenite type compound wherein the cages include only electrons is heated to high temperature such as 500° C. in air, oxygen in the atmosphere changes to oxygen ions due to electrons donated from the cages, and a reaction of introducing the oxygen ions into the cages easily occurs. The reason is that since the framework forming the cage structure of the mayenite compound has a positive charge, oxygen ions having a negative charge are introduced into the cages, but oxygen molecules or oxygen atoms, which are electrically neutral, are less likely to be introduced.
Therefore, as compared with a mayenite type compound wherein only electrons are included, a mayenite type compound having a low electron-donating property to oxygen in air is capable of suppressing the reaction of generating oxygen ions from oxygen molecules and thereby lowering the introducing rate of oxygen ions to the cages.
As a method to obtain such a mayenite type compound, Patent Document 3 discloses a method wherein first, a mayenite type compound is synthesized at a temperature of from 1,200 to 1,415° C., and then it is subjected to hydrogen reduction treatment to obtain a mayenite type compound wherein hydride ions are included.